1. Field of the invention
The present invention relates to a linear image sensor, and more specifically to a layout of bonding pads and location of bonding stitches in the linear image sensor.
2. Description of related art
In conventional linear image sensors, an image sensor chip, which is generally formed of a semiconductor substrate, has an active area or circuit region including a number of photosensitive elements arranged and aligned in a single straight array. In a typical example, the size of the circuit region is 300 .mu.m.times.30,000 .mu.m in a plan view. Namely, the circuit region is extremely oblong. On the other hand, the required number of bonding pads to be formed on the chip is relatively small. Therefore, if the bonding pads were located and distributed at a periphery of the circuit region, a substantial invalid or empty area inevitably occur in a peripheral region of the circuit region.
Japanese Patent Application Laid-open No. Hei 2-065278 published on Mar. 5, 1990 shows some layouts of various elements within the linear image sensor chip for the purpose of making the empty area in the chip as small as possible. In one shown layout, the chip is formed of an elongated semiconductor substrate, and the circuit region including a number of photosensitive elements is provided at a center zone of the chip in a longitudinal direction of the elongated chip. Bonding pads are divided into two groups, which are locally arranged at opposite end zones of the elongated chip, respectively. Since the two groups of bonding pads are concentrated in the opposite end zones of the elongated chip, the empty area of the periphery of the circuit region on the chip can be reduced to some degree.
However, in each opposite end zone of the elongated chip, the bonding pads are arranged in two rows, which are separated from each other in a transverse direction perpendicular to the longitudinal direction of the elongated chip. Therefore, the two rows of the bonding pads require a substantial width, which is considerably larger than the width of the circuit region, with the result that a substantial empty area remains at each side of the circuit region along the longitudinal direction of the circuit region.
On a package substrate for supporting the sensor chip thereon, there are provided a number of bonding stitches to be connected to the bonding pads of the sensor chip through bonding wires. A minimum pitch of the bonding stitches is considerably larger than a minimum pitch of the bonding pads, and this difference in pitch has restricted the location of the bonding pads and the bonding stitches, with the result that the reduction of the length and width of the sensor chip is limited. This means that a substantial empty area still remains in the elongated chip. In addition, the difference in pitch has required a long bonding wire, and in extreme cases, makes it difficult to connect between the bonding pads and the bonding stitches by the bonding wires.